It is known that many internal combustion engines, including spark-ignition engines (e.g. gasoline engines), are commonly provided with a particulate filter for trapping most of the particulate matter (soot) produced by the engine in order to reduce the polluting emissions.
The soot collected inside the particulate filter is removed from time to time by means of a soot combustion process, generally referred to as the regeneration process, which takes place when the temperature of the particulate filter is above a certain value (e.g. 500° C.) and enough oxygen is present in the exhaust system.
On engines with homogeneous stoichiometric combustion, such as gasoline engines, the regeneration processes can mainly occur during fuel cut off phases, when no fuel is supplied into the engine cylinders and large amounts of oxygen are pumped towards the particulate filter by the engine pistons. Under these conditions, if the temperature of the particulate filter reaches the prescribed value, the combustion of the collected soot occurs spontaneously (passive regeneration).
However, during short driving cycles or driving cycles with low load operations, passive regeneration could be impossible either because the temperature of the particulate filter is unable to reach the triggering value, or because no fuel cut off phases are performed. To avoid overloading of the particulate filter under these scenarios, an active regeneration process is needed when the amount of soot collected inside the particulate filter exceeds a predetermined threshold value thereof.